The present invention relates to a molding apparatus of the type that a thixotropic alloy material is heated substantially semi-molten, and filled in the cavity of a mold, to form a desired product, and more particularly to a molding apparatus which is so designed that an oxide formed on the surface of the material is prevented from mixing in the molding, and to an improvement of a molding material supplying device in the apparatus.
In addition, the present invention relates to a manufacturing method which is suitable for producing wheels of light alloy for automobiles or motor-bicycles, and an apparatus for practicing the method, and more particularly to a method of heating the thixotropic alloy material.
Recently, light alloys such as aluminum alloys and magnesium alloys have been extensively employed as materials for forming mechanical products such as automobile components and office equipment. The light alloys are, in general, formed by die casting or other casting methods.
In a conventional casting method, a metal material is heated higher than the liquidus so that the molten material is poured into the mold. Hence, when the molten material is cooled in the mold, dendrites grow in the material, thus lowering the mechanical strength of the molding or causing defects therein. If, in the casting method, a molding material liable to explode at temperatures near the liquidus is employed, then it must be molten in an inactive atmosphere. As a result, an apparatus for practicing the method and accordingly its operation are unavoidably intricate.
With a conventional semi-molten metal molding apparatus, a molding is formed as follows: That is, as shown in FIG. 6, a metal billet B which has been semi-molten by heating, is pressed against a die D by a punch P, so as to flow through a runner R into a metal mold M to fill the cavity C with the metal billet B, to form a molded article into a desired shape.
In the molding operation, the billet B is heated at high temperature to be semi-molten as described above. Hence, while being moved from a heating device (not shown) to the mold, the billet contacts the air, so that a thick oxide film S is formed on the surface of the billet. When the billet B is pushed out of the die D with the punch P, the oxide film S is caused to flow together with the billet body to the cavity C, where it is mixed in the molding. The oxide film thus mixed lowers the mechanical strength or quality of the molding depending on its nature.
The above-described difficulties may be eliminated by employing the following method: The heating of the billet, and the movement of the billet thus heated to the die D are carried out in an inactive atmosphere so that it may not contact the air. However, if an inactive atmosphere is provided at all the places where the billet is heated and moved as described above, then it will make the apparatus bulky and expensive, and increase the manufacturing cost of the molding.
One of the difficulties involved in the practicing of the above-described method is that, after being heated semi-molten at high temperature, the light metal must be quickly molded with its temperature maintained unchanged. That is, if the temperature of the material is excessively high during molding, then primary crystals formed in the material are coarse, so that the resultant product is low in mechanical characteristic, or the liquid phase fraction is high, so that it is not suitably semi-molten, and furthermore the material at high temperature is greatly oxidized to form an oxide film on the surface thereof, which is liable to mix in the molding. If, on the other hand, the temperature of the material is excessively low, then it is impossible for the punch to apply a sufficiently high pressure to the material, so that the resultant molding may be incomplete.
The above-described method of molding a semi-molten light alloy into a wheel for automobiles or motor-bicycles, and the apparatus for practicing the method are well known in the art (see, for example, Examined Japanese Patent Application Publication No. 5748/1983 or Unexamined Japanese Patent Application (OPI) No. 19499/1980).
However, there are some problems to be solved in the method, and it is not practically in use yet. One of the reasons for this fact is that the material is not high enough in yield. Roughly stated, there are two reasons for this poor yield. One of the reasons resides in defects which are formed during molding; that is, impurities are mixed in the molding or shrinkage cavities are formed therein. The surface of the material flowing in the cavity of the mold is covered with a relatively thick oxide film. Hence, when the flow of material branches in the cavity, or the flows of material meet together, the oxide film is mixed inside the material. The other reason is that the gate member is large. The gate is extended outwardly from the cavity, and in order to facilitate the cutting of the gate member, a small diameter portion is necessary to be formed at the portion where it is connected to the cavity. Thus, the gate is relatively long.
One example of the method of heating the material is a so-called "rheocasting method" in which, in order to eliminate casting defects or difficulties accompanying production technique, a metal material is made molten and then cooled until it shows solid and liquid phases, and under this condition the metal material is sufficiently stirred by a mechanical method or electromagnetic induction method, to stop the growth of dendrites, or to finely break them, and the material is casted after such a process is performed (see, for example, Unexamined Japanese Patent Application (OPI) No. 152358/1985). On the other hand, in the case of using a metal material such as a magnesium alloy which is oxidized substantially at the melting temperature, the employment of the rheocasting method is not practical, being applicable only to a magnesium foundry, causing other methods to be employed. That is, a so-called "thixoforging method" is employed in which a billet casted by rheocasting is heated again until it shows both solid and liquid phases, and the billet thus processed is pressed and filled in the mold.
As described above, the thixoforging method uses the billet formed according to the rheocasting method. Hence, the thixoforging method is advantageous in that a forging, excellent in quality, can be formed without using a high temperature oven for melting a material; however, it is disadvantageous in that it needs a large number of manufacturing steps when counted from the billet casting step, which increases the manufacturing cost of the product.